1. Field of the Invention
This invention relates to optionally solventless two-component polyurethane surfacers containing finely divided fillers in a high pigment volume concentration based on a polyfunctional isocyanate component (B) and a polyol component (A) which is liquid at room temperature and which consists of mixture components containing isocyanate-reactive hydroxyl groups with an at least predominant content of branched-chain polyols (A1) containing ester and ether groups, which contains at least a proportion of the finely divided fillers and, optionally, the additives and which, for application, is mixed with the isocyanate component(B) and applied as a mixture that can still be processed by shaping. The present invention also relates to the use of the surfacers according to the invention.
2. Statement of Related Art
In practice, various surfacers or filling materials are used for repairing structural components of a variety of materials where losses of material have to be replaced. These surfacers or filling materials differ according to the type of binder used. Aqueous polymer dispersions, two-component epoxy resins, two-component polyurethane systems, two-component polyester systems and oxidatively crosslinking oil or alkyd resins are typically used for surfacers of the type in question. Unfortunately, surfacers of this type show inadequate adhesive properties, particularly if the thickness of the joint is of the order of one tenth of a millimeter or more. In addition, they are extremely vulnerable to cracking on account of different expansion coefficients in relation to the material to be repaired and tend to separate or even crack in the event of temperature fluctuations in the flank region.
On the other hand, there are several adhesives, partly based on the binders already mentioned, which--although suitable for joining substrates--can only be used with considerable limitations, if at all, as filling materials. Thus, polyvinyl acetate glues for example are mainly used for bonding wood and wood materials. However, the use of these adhesives presupposes an exact fit of the joint. Gaps cannot be filled with these adhesives, inter alia on account of their considerable shrinkage. In addition, phenol-resorcinol resins are used to a limited extent; although they can readily be filled, they are not suitable as proper surfacers.
Moisture-curing one-component polyurethane systems are also being used to an increasing extent. These products show more or less pronounced foaming and, by virtue of this increase in volume, have gap-filling properties, but are not suitable as surfacers because the necessary strengths cannot be reached on account of the foam structure, besides which the rheology of these systems is generally such that adequate firmness, for example on vertical surfaces, cannot be achieved.
Generally, it may be said that, in contact to adhesives, surfacers have a higher ratio of fillers to binder (pigment volume concentration, PVC), inter alia to avoid high shrinkage. If these surfacers predominantly contain inorganic fillers, they are generally difficult to machine with the tools normally used, for example by grinding, sawing, drilling, etc., and cause heavy tool wear. The effect of the high pigment volume concentration is that, in general, surfacers of the type in question are not good adhesives.
In the woodwork field, there is some incentive to develop filling surfacers having improved adhesive properties, for example based on epoxy resins, as described in DE 26 06 138. In addition, products based on a two-component polyurethane system are available on the market, but are attended by a number of significant disadvantages, beginning for example with poor handling properties during stirring of the two components, i.e., the mixture is very difficult to homogenize. In addition, many products cannot be spread out to thin layers over a homogeneous, non-porous surface. In addition, firmness on vertical or overhead surfaces is inadequate and/or curing is accompanied--through the evolution of CO.sub.2, which in one case is probably caused by reaction of the isocyanate component with the residual moisture in the wood chips present as filling material--by an unwanted increase in volume which, in addition, seriously impairs internal strength through the porous foam structure formed. On the other hand, CO.sub.2 can be formed in undesirably large amounts by reaction with ambient and/or substrate moisture, even when NCO groups are present in a distinct stoichiometric excess, i.e., when the isocyanate component is heavily overdosed. Products such as these are no solution to the problem in cases where losses of material, for example from wood or wood components, have to be replaced and, at the same time, high strength requirements have to be satisfied, as in the repair of broken hinges on doors or in the replacement of heavily stressed components which have been destroyed or functionally impaired by rotting, fungal attack, corrosion, material fatigue or the like. Polyurethane surfacers are not normally used for special substrates, such as metals.
The above-mentioned wood surfacers based on two-component polyurethanes may be produced, for example, from polyether polyols filled with sawdust or wood chips as the polyol component (A) and a polyfunctional isocyanate component (B), for example diphenyl methane diisocyanate (MDI). Processability is often impaired on account of the high filler content and/or the size of the filler particles.
To sum up, it may be said that the various requirements which an adhesive compound on the one hand and a surfacer on the other hand have to meet in one and the same product have not hitherto been satisfied without serious restrictions. Accordingly, there is still a need in industry, commerce, and the workshop for multipurpose products on the one hand, i.e., products which can be used for filling or bonding, and on the other hand for products having wider potential applications, i.e., filling and bonding, as required for example for the replacement of material in heavily stressed structural components.